Glow plugs and installation in the engine head



Nov. 2, 1965 F. TEsTERlNl 3,215,131

GLOW PLUGS AND INSTALLATION IN THE ENGINE HEAD Original Filed Deo. l2,1961 INVENTOR. 'francesa 7225 er-m r' war/Ei@ @.,PA/

wol/ral United States Patent O 3,215,131 GLOW PLUGS AND INSTALLATION INTHE ENGINE HEAD Francesco Testerini, Milan, Italy, assignor to FabricaItaliana Magneti Marelli S.p.A., Milan, Italy, a corporation of ItalyContinuation of abandoned application Ser. No. 158,768, Dec. 12, 1961.This application Mar. 16, 1964, Ser. No. 352,157

Claims priority, application Italy, Dec. 23, 1960, 21,970/ 60 Claims.(Cl. 123-169) This invention relates to ignition spark plugs forinternal combustion engines of the glow plug type, also known assemi-superficial or surface-gap type, and more particularly it relatesto spark plugs -of the type wherein the spark grazes the surface of theinsulator comprising a center electrode surrounded about its point by athin insulating medium and substantially enclosed by a metal bodyelectrode concentric with the surrounded central electrode and havingradial grooves or openings communicating between the enclosed spark plugchamber and the combustion chamber of the engine. In this type of plugthe spark grazes the insulator as it crosses the annular gap formedbetween the center electrode and the metal body electrode.

lrl`he invention relates, further, to the manner of installation ormounting of said spark plugs in the engine head. In conventional sparkplugs, the body of the plug constitutes one grounded electrode which isscrewed into the engine head to a limiting position where the centerelectrode with insulator projects substantially into the combustionchamber beyond the engine head. Because of this projection, Vwhen thespark plug becomes hot, the ignition acts as it would with a sparkadvance, thus changing the engine timing characteristics. Part of thisdisadvantage results from the fact that the limited projecting plug areamust conduct heat to the point that the plug contacts the engine headwhere the conduction path is not so limited by a small heat transmissionarea.

Also, the spark gap spacing may not be held constant in prior art plugs,because of imperfect centering of the insulator in the plug body, wherethe centering is dependent upon a rim operation and locking by means ofa threaded ring. When imperfectly centered, the sparking is localizedwith resulting pitting and reduced life -of the plug.

Furthermore, diiculties are encountered in the milling and manufactureof Ventilating slits to produce exact shape and conformation with thecenter electrode, thereby making the plugs expensive.

It is an object of the present invention to provide an improvedinstallation of a glow plug in an engine, thereby resolving the abovementioned problems.y

Spark plugs of the present invention comprise a central electrodeassembly substantially enclosed by the body which comprises the outergrounded elect-rode, and which contains radially disposed ventilationslits for communication between the spark plug chamber and thecombustion chamber. These radially extending slits are arrangedsymmetrically above the axis of the plug to allow a homogeneousinterchange of the interior gases.

A threaded plug body is provided of such length that the plug cannot beinserted to project beyond the interior engine surface into thecombustion chamber, but is mounted entirely within the engine block orhead. Thus, the plug is linstalled with its point either recessed orpreferably in the same plane with the internal surface of the enginewith the ends of the center electrode, insulator and the body on thesame level.

The invention will now be described by way of exam- Patented Nov. 2,1965 "ice ple, with reference to the preferred embodiments of theaccompanying drawing, wherein:

FIGURE 1 is a plan view of the electrode structure of a plug constructedin accordance with the invention,

FIGURE 2 is a side view, partly in section of a plug installed in anengine in accordance with the principles of the invention, and

FIGURE 3 is a fragmentary View, partly in section of a further plugembodiment of the invention.,

The spark plug comprises an insulator 1 surrounding center electrode 2within body 3, which is inserted in the engine head 4 by means of screwthreads on the threaded end portion 3 of the body as limited by theouter sealing gasket 6.

An annular plug chamber 5 is formed within the end portion 3 by thespace between tapered end of insulator l1, having a thin insulatingcovering 8 extending out to the end of the center electrode 2, and theinner surface of the body member 3.

In order to secure suiiicient ventilation of the chamber 5 enclosed bythe inwardly flared portion 3" of the body, radially extended grooves 7are formed therein by a broach in a symmetrical pattern about the axisof the plug or the center electrode 2. The inwardly iiared portion 3" ofthe body has a central aperture positioned concentrically about the thininsulating covering 8 of the center electrode 2, from which the grooves7 extend. Thus, gas movement is facilitated to produce a homogeneouschange of gases within the chamber 5 to provide Icooling for theinsulator portion 8 which is heated until it glows when grazed by sparkspassing between the two electrodes.

To permit good centering of the thin insulator portion 8 within theinwardly ared portion 3" of the body, a centering tool (not shown) isinserted in all the grooves 7 during assembly to lock the position ofthe insulator 1 into position. This assures uniform distribution of thespark, which, with the better cooling provided by conduction of heatthrough the head 4 in Contact with the entire threaded end portion 3',assures a long wearing life of the spark plug. The inwardly flaredportion 3" is preferably integral with the body 3 to provide betterthermal conduction to the engine head 4.

As shown in FIGURES 2 and 3, the end of the spark plug is mounted Hushwith the interior surface 4' of the engine. Thus, the end of the centerelectrode 2, the thin insulation covering 8, and the inwardly flaredportion 3" of the body are all at the same level in the plane of theinner surface 4 of the engine as shown in FIGURE 2. This causes thespark between the two electrodes to graze the end portion 8 of theinsulator, whereby the plug operates on the glow-plug principle.

In FIGURE 3, the electrode 2, inwardly flared section 3 and insulatorcovering 8 are recessed within the plug, while the threaded end portion3 is preferably in the same plane as the internal surface 4 of theengine. It is important that no part of the plug protrudes inside thecombustion chamber after installation. Thus, the length of the threadedportion 3 of the plug body is limited.

Thus, improved performance is afforded by the novel constructionfeatures of the present glow-type spark plug with the manner ofinstallation within the engine head, as defined in the following claims:

1. An ignition spark plug with a spark path partly grazing the surfaceof its insulator comprising a center electrode surrounded to its tip bya thin insulator, an outer electrode comprising a metal body portionconcentrically surrounding the insulator and terminating in a planeconfined within the end of the plug and common with the end of thecenter electrode and the end of its surrounding insulator to cause,thereby, the spark between the electrodes to graze the insulator, anannular chamber between said body and said insulator, and radiallyextending symmetrical passages in the metal body about said centerelectrode communicating with the chamber thereby to provide homogeneousinterchange of gases within said chamber wherein the outer electrode isan integral portion of a plug body member provided with mounting threads-for insertion into an engine head which has the end of the body flaredinwardly.

2. A plug, as defined in claim 1, wherein the metal body is providedwith mounting threads having a limiting position such that the plug maybe inserted within a standard engine mount without projecting into thecornbustion chamber.

3. A radial discharge ignition spark plug in an engine comprising acenter electrode surrounded by a thin insulator and a metal ybody spacedfrom the insulator in the region yof the end of said insulator, saidplug having an annular chamber between said body and said insulator andpassages communicating with said chamber and the combustion chamber ofthe engine, characterized in that said passages extend at least throughthe portion of the body of the spark plug inside said engine head and inthat the center electrode is recessed with respect to the end of thebody.

4. A11 ignition spark plug according to claim 3, characterized in thatthe end of the body is ared inwardly.

5. For installation in an engine head, a plug with a sparking pathdefined to graze the surface of an insulator interposed between twoelectrodes and having a body for insertion in said head with itsterminal end at a limiting position flush with the inner wall of acombustion chamber in said engine head, a center electrodeconcentrically surrounded by an insulation layer, within said body andan outer electrode concentrically surrounding said insulation layer,wherein the electrode, body and insulator terminate at a single planeperpendicular to the center electrode and the outer electrodeconstitutes an end portion of the plug body which is ared inwardly andrecessed from the end of the plug.

References Cited by the Examiner UNITED STATES PATENTS 1,368,358 2/21Schwetger. 2,010,991 8/ 35 Hornback. 2,129,576 9/ 38 Gorny et al.

2,899,585 8/59 Dollenberg 313--143 3,016,411 1/62 Fisher 123-169 FOREIGNPATENTS 527,311 5/55 Italy.

RICHARD B. WILKINSON, Primary Examiner.

1. AN IGNITION SPARK PLUG WITH A SPARK PATH PARTLY GRAZING THE SURFACEOF ITS INSULATOR COMPRISING A CENTER ELECTRODE SURROUNDED TO ITS TIP BYA THIN INSULATOR, AN OUTER ELECTRODE COMPRISING A METAL BODY PORTIONCONCENTRICALLY SURROUNDING THE INSULATOR AND TERMINATING IN A PLANECONFINED WITHIN THE END OF THE PLUG AND COMMON WITH THE END OF THECENTER ELECTRODE AND THE END OF ITS SURROUNDING INSULATOR TO CAUSE,THEREBY, THE SPARK BETWEEN THE ELECTRODES TO GRAZE THE INSULATOR, ANANNULAR CHAMBER BETWEEN SAID BODY AND SAID INSULATOR, AND RADIALLYEXTENDING SYMMETRICAL PASSAGES IN THE METAL BODY ABOUT SAID CENTERELECTRODE COMMUNICATING WITH THE CHAMBER THEREBY TO PROVIDE HOMOGENEOUSINTERCHANGE OF GASES WITHIN SAID CHAMBER WHEREIN THE OUTER ELECTRODE ISAN INTEGRAL PORTION OF A PLUG BODY MEMBER PROVIDED WITH MOUNTING THREADSFOR INSERTION INTO AN ENGINE HEAD WHICH HAS THE END OF THE BODY FLAREDINWARDLY.